A fresh look at sleep needs shows little evolution

Study reports similar patterns for Americans, hunter-gatherers

For years, public health authorities have warned that smartphones, television screens and the hectic pace of modern life are disrupting natural sleep patterns, fueling an epidemic of sleep deprivation. By some estimates, Americans sleep two to three hours less today than they did before the Industrial Revolution.

But a new study is challenging that notion. It found that Americans, on average, sleep as much as people in three hunter-gatherer societies where there is no electricity and lifestyles have remained largely the same for thousands of years. If anything, the hunter-gatherer communities in the new study — the Hadza and San tribes of Africa, and the Tsimané people of South America — slept even less than many Americans.

The health authorities have long suggested that consistently having a minimum of seven hours is a necessity for good health. Many studies suggest that lack of sleep, independent of other factors such as physical activity, is associated with obesity and chronic disease.

Yet the hunter-gatherers in the new study, published in Current Biology, were relatively fit despite regularly sleeping for periods near the low end of those in industrialized societies. Previous research shows that their daily energy expenditure is about the same as most Americans,’ suggesting physical activity is not the reason for their relative good health.

The prevailing notion in sleep medicine is that humans evolved to go to bed when the sun went down, and that, by and large, we stay up much later than we should because we are flooded with artificial light, said Jerome Siegel, lead author of the new study and a professor of psychiatry at the University of California at Los Angeles.

But Siegel and his colleagues found no evidence of this. The hunter-gatherer groups they studied, which slept outside or in crude huts, did not go to sleep when the sun went down. Usually, they stayed awake three to four hours past sunset, with no light exposure other than the glow of a small fire to keep animals away and provide a bit of warmth in the winter.

In a typical night, they slept just six and a half hours — slightly less than the average American. In the United States, most adults sleep seven hours or more a night, though a significant portion of the population sleeps less.

Nathaniel Watson, the president of the American Academy of Sleep Medicine, pointed out that the hunter-gatherer societies were found to have a sleep period — meaning the time they were actually in bed — of roughly seven to eight and a half hours, which he said was consistent with his group’s recommendations.

The question of how much sleep people require was a delicate one, he said. “Really it’s just the amount that allows people to wake up feeling refreshed and alert.”

Many researchers argue that the invention of the electric light bulb in the late 1800s drastically changed sleep. Exposure to artificial light at night, whether from light bulbs or computer screens, throws off the body’s biological clock, delaying and reducing sleep, these experts believe.

Some historians have also argued that it is not natural for people to sleep straight through the night. They say that before the introduction of artificial light,it was normal for people to sleep in two intervals separated by an hour of wakefulness, a phenomenon known as segmented sleep, or “first” and “second” sleep.

But Siegel said he questioned those assertions because there were no rigorous studies of sleep behaviors. He and his colleagues decided that one way to gain some insight was to study cultures relatively unaffected by artificial light.

Among those they chose to follow were the Hadza people, who spend their days hunting and foraging in northern Tanzania, much as their ancestors have for tens of thousands of years; the San of Namibia, who have lived as hunter-gatherers in the Kalahari for at least 20,000 years; and the Tsimané, a seminomadic group that lives in the Andean foothills of Bolivia, near the farthest reaches of the human migration out of Africa. Members of the various tribes were fitted with small devices that tracked their sleep patterns and their exposure to light across the seasons.

The researchers found that in addition to sleeping roughly similar amounts each night, the three groups rarely took naps during the day and did not sleep in two separate intervals at night.

“The Hadza and the San live in the area where we know humans evolved, and then the Tsimané live in some sense at the end of the human migration,” he said. “The fact that we see very similar sleep times gives me great confidence that this is how all of our ancestors slept.”

Their sleep did not seem to be problematic. Chronic insomnia, which affects 20 percent to 30 percent of Americans, occurred in just 2 percent of the hunter-gatherers. The San and the Tsimané did not even have a word for it in their languages.

The groups did not go to sleep at sunset and they did not wake up at sunrise, suggesting that light exposure did not have much influence on their sleep patterns. But they almost always fell asleep as the temperature began to fall at night, and they would wake up as the temperature was rising again. This suggests that humans may have evolved to sleep during the coldest hours of the day, perhaps as a way to conserve energy, Siegel said.